How can I add a link once so it appears throughout the forums?

I would like to add, let’s say a link, at the top of all bbPress pages. In which file can I do this so I don’t have to be adding the same piece of code in every file manually, and only in the bbpress, not throughout the actual site.

Thanks, but what I was trying to say is: if I want to add a banner or a piece of code to the bbPress theme, in which file can I paste the code so it shows in the whole forum’s site? So I don’t have to repeat the code over and over.